How do you evaluate the expression \[4ab\], for \[a=2\] and \[b=5\] ?

Hint: For solving this question we have to consider the given algebraic expression as equation (1) and then start solving the question by one by one step process. First of all substitute value of ‘a’ in expression and next substitute value of ‘b’ in expression to get the final result.

Complete step-by-step answer:
For the given problem, we are given to evaluate the expression \[4ab\], for \[a=2\] and \[b=5\].
For evaluating the given expression let us assume the expression as equation (1).
\[x=4ab..........\left( 1 \right)\]
Now, from the question we have to substitute the value of \[a=2\] and \[b=5\] in the equation (1) then we will get the value.
Let us substitute the value of \[a=2\] in equation (1), we get
\[\Rightarrow x=4.\left( 2 \right).\left( b \right)\]
By simplifying a bit we will get
\[\Rightarrow x=8b\]
Let us consider the above equation as equation (2).
\[x=8b............\left( 2 \right)\]
Now for evaluating the equation we have to substitute the value \[b=5\] to get the final answer.
Now we have to substitute the value in equation \[b=5\], then we will get the solution
By substituting the value \[b=5\] in equation (2), we get
\[\Rightarrow x=8\left( 5 \right)\]
By simplifying a bit we will get
\[\Rightarrow x=40\]
Let us consider the above equation as equation (3).
\[x=40............\left( 3 \right)\]
Therefore by evaluating the expression \[4ab\], with \[a=2\] and \[b=5\] we will get \[x=40\].

Note: We can solve this problem by substituting ‘a’ and ‘b’ at a time in the equation (1). Examiner may give negative values also for example: evaluate the expression \[4ab\], for \[a=-2\] and \[b=5\]. If one of the values is negative then the whole result will be negative. If both ‘a’ and ‘b’ values are negative then the result will be positive.